1. Field of the Invention
This invention relates to an elastic surface wave element useful as a delay line, an oscillator, a filter, etc.
2. Description of the Prior Art
The elastic surface wave element has heretofore been utilized in special military applications. In recent years, it has come to be used in such civilian devices as FM tuner and TV and has been arresting rapidly growing attention. Specifically, elastic surface wave elements have been reduced to such commercial products as delay elements, oscillators, and filters. The elastic surface wave elements of various types are characterized by compactness, light weight, and high reliability and by the fact that the process for their production is similar to that for the production of integrated circuits and, therefore, abounds in mass producibility. At present, they are being mass produced as indispensable electronic parts.
A typical conventional elastic surface wave element will be described below as embodied in an elastic surface wave rexonator. As illustrated in FIG. 7 and FIG. 8, an electrode 2 formed of an electroconductive substance after the pattern of a grille is deposited on a piezzoelectric substrate 1. In this case, the piezzoelectric substrate 1 is formed of a piezzoelectric single crystal or piezzoelectric ceramic such as of quartz or lithium niobate or a piezzoelectric film superposed on the surface of glass. The grille-shaped electrode 2 can be formed by vacuum depositing a metal such as aluminum or gold on the surface of the piezzoelectric substrate 1 and photoetching the deposited layer of metal in a prescribed pattern. A pair of latticed reflectors 3, 3 each composed of a dielectric, a conductor, grooves, etc. are formed on each on both sides of the grille-shaped electrode 2.
When a voltage of a specific frequency is applied on the grille-shaped electrode 2, an electric field is generated on the surface of the piezzoelectric substrate 1 in the spaces of the grille-shaped electrode 2 and, owing to the piezzoelectricity of the piezzoelectric substrate 1, strain is produced proportionately to the magnitude of the applied voltage. This strain is propagated as surface wave in both directions at a speed of sound fixed by the kind of material of the piezzoelectric substrate 1. The surface wave is reflected by the latticed reflectors 3, 3 disposed on both sides and returned to the grille-shaped electrode 2 to induce resonance.
Generally, any elastic surface wave element of this class is tightly enclosed in a metallic container called a hermetic seal 4 and constructed as illustrated in FIG. 9. In due consideration of the sealing property and the resistance to corrosion, the hermetic seal 4 is plated with a nickel coating, for example.
The conventional elastic surface wave element of the foregoing construction, however, suffers from the disadvantage that electroconductive foreign matter which has found its way to the elastic surface wave element before the element is tightly enclosed in the hermetic seal 4 or part of the nickel plating peeling off the surface of the hermetic seal adheres to the grille-shaped electrode and induce the phenomenon of short-circuit between the electrode bars. The short-circuit brings about various hindrances such as variation in electric impedance. This disadvantage impairs the reliability of the elastic surface wave element and interferes with mass production of such surface wave elements.